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  1. Free, publicly-accessible full text available June 1, 2025
  2. Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and plays a pivotal role in climate, air quality, and health. The production of low-volatility dimeric compounds through accretion reactions is a key aspect of SOA formation. However, despite extensive study, the structures and thus the formation mechanisms of dimers in SOA remain largely uncharacterized. In this work, we elucidate the structures of several major dimer esters in SOA from ozonolysis of α-pinene and β-pinene—substantial global SOA sources—through independent synthesis of authentic standards. We show that these dimer esters are formed in the particle phase and propose a mechanism of nucleophilic addition of alcohols to a cyclic acylperoxyhemiacetal. This chemistry likely represents a general pathway to dimeric compounds in ambient SOA. 
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    Free, publicly-accessible full text available November 17, 2024
  3. Abstract. This study describes a modeling framework, model evaluation, and source apportionment to understand the causes of Los Angeles (LA) air pollution. A few major updates are applied to the Community Multiscale Air Quality (CMAQ) model with a high spatial resolution (1 km × 1 km). The updates include dynamic traffic emissions based on real-time, on-road information and recent emission factors and secondary organic aerosol (SOA) schemes to represent volatile chemical products (VCPs). Meteorology is well predicted compared to ground-based observations, and the emission rates from multiple sources (i.e., on-road, volatile chemical products, area, point, biogenic, and sea spray) are quantified. Evaluation of the CMAQ model shows that ozone is well predicted despite inaccuracies in nitrogen oxide (NOx) predictions. Particle matter (PM) is underpredicted compared to concurrent measurements made with an aerosol mass spectrometer (AMS) in Pasadena. Inorganic aerosol is well predicted, while SOA is underpredicted. Modeled SOA consists of mostly organic nitrates and products from oxidation of alkane-like intermediate volatility organic compounds (IVOCs) and has missing components that behave like less-oxidized oxygenated organic aerosol (LO-OOA). Source apportionment demonstrates that the urban areas of the LA Basin and vicinity are NOx-saturated (VOC-sensitive), with the largest sensitivity of O3 to changes in VOCs in the urban core. Differing oxidative capacities in different regions impact the nonlinear chemistry leading to PM and SOA formation, which is quantified in this study.

     
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    Free, publicly-accessible full text available January 1, 2025
  4. Increased wildfire events constitute a significant threat to life and property in the United States. Wildfire impact on severe storms and weather hazards is another pathway that threatens society, and our understanding of which is very limited. Here, we use unique modeling developments to explore the effects of wildfires in the western US (mainly California and Oregon) on precipitation and hail in the central US. We find that the western US wildfires notably increase the occurrences of heavy precipitation rates by 38% and significant severe hail (≥2 in.) by 34% in the central United States. Both heat and aerosols from wildfires play an important role. By enhancing surface high pressure and increasing westerly and southwesterly winds, wildfires in the western United States produce ( 1 ) stronger moisture and aerosol transport to the central United States and ( 2 ) larger wind shear and storm-relative helicity in the central United States. Both the meteorological environment more conducive to severe convective storms and increased aerosols contribute to the enhancements of heavy precipitation rates and large hail. Moreover, the local wildfires in the central US also enhance the severity of storms, but their impact is notably smaller than the impact of remote wildfires in California and Oregon because of the lessened severity of the local wildfires. As wildfires are projected to be more frequent and severe in a warmer climate, the influence of wildfires on severe weather in downwind regions may become increasingly important. 
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  5. Abstract. A new technique was used to directly measure O3 response to changes inprecursor NOx and volatile organic compound (VOC) concentrations in the atmosphere using threeidentical Teflon smog chambers equipped with UV lights. One chamberserved as the baseline measurement for O3 formation, one chamber addedNOx, and one chamber added surrogate VOCs (ethylene, m-xylene,n-hexane). Comparing the O3 formation between chambers over a3-hour UV cycle provides a direct measurement of O3 sensitivity toprecursor concentrations. Measurements made with this system at Sacramento,California, between April–December 2020 revealed that theatmospheric chemical regime followed a seasonal cycle. O3 formation wasVOC-limited (NOx-rich) during the early spring, transitioned toNOx-limited during the summer due to increased concentrations ofambient VOCs with high O3 formation potential, and then returned toVOC-limited (NOx-rich) during the fall season as the concentrations ofambient VOCs decreased and NOx increased. This seasonal pattern ofO3 sensitivity is consistent with the cycle of biogenic emissions inCalifornia. The direct chamber O3 sensitivity measurements matchedsemi-direct measurements of HCHO/NO2 ratios from the TROPOsphericMonitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor (Sentinel-5P) satellite. Furthermore, the satellite observations showed thatthe same seasonal cycle in O3 sensitivity occurred over most of theentire state of California, with only the urban cores of the very largecities remaining VOC-limited across all seasons. The O3-nonattainmentdays (MDA8 O3>70 ppb) have O3 sensitivity in theNOx-limited regime, suggesting that a NOx emissions controlstrategy would be most effective at reducing these peak O3concentrations. In contrast, a large portion of the days with MDA8 O3concentrations below 55 ppb were in the VOC-limited regime, suggesting thatan emissions control strategy focusing on NOx reduction would increaseO3 concentrations. This challenging situation suggests that emissionscontrol programs that focus on NOx reductions will immediately lowerpeak O3 concentrations but slightly increase intermediate O3concentrations until NOx levels fall far enough to re-enter theNOx-limited regime. The spatial pattern of increasing and decreasingO3 concentrations in response to a NOx emissions control strategyshould be carefully mapped in order to fully understand the public healthimplications. 
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  7. Abstract. Particle size measurement in the low nanometer regime is of great importance to the study of cloud condensation nuclei formation and to better understand aerosol–cloud interactions. Here we present the design, modeling, and experimental characterization of the nano-scanning electrical mobility spectrometer (nSEMS), a recently developed instrument that probes particle physical properties in the 1.5–25 nm range. The nSEMS consists of a novel differential mobility analyzer and a two-stage condensation particle counter (CPC). The mobility analyzer, a radial opposed-migration ion and aerosol classifier (ROMIAC), can classify nanometer-sized particles with minimal degradation of its resolution and diffusional losses. The ROMIAC operates on a dual high-voltage supply with fast polarity-switching capability to minimize sensitivity to variations in the chemical nature of the ions used to charge the aerosol. Particles transmitted through the mobility analyzer are measured using a two-stage CPC. They are first activated in a fast-mixing diethylene glycol (DEG) stage before being counted by a second detection stage, an ADI MAGIC™ water-based CPC. The transfer function of the integrated instrument is derived from both finite-element modeling and experimental characterization. The nSEMS performance has been evaluated during measurement of transient nucleation and growth events in the CLOUD atmospheric chamber at CERN. We show that the nSEMS can provide high-time- and size-resolution measurement of nanoparticles and can capture the critical aerosol dynamics of newly formed atmospheric particles. Using a soft x-ray bipolar ion source in a compact housing designed to optimize both nanoparticle charging and transmission efficiency as a charge conditioner, the nSEMS has enabled measurement of the contributions of both neutral and ion-mediated nucleation to new particle formation.

     
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